1. Field of the Invention
The present invention relates to an optical scanning apparatus, and more particularly to an optical scanning apparatus for scanning a plurality of beams modulated in accordance with image data on receiving surfaces by use of a single deflector.
2. Description of Related Art
In recent years, in the field of image forming apparatuses, such as full-color copying machines, printers, etc., a tandem type is general. In the tandem type, four photosensitive members respectively for Y (yellow), M (magenta), C (cyan) and K (black) are arranged in parallel, and images in the respective colors formed on the photosensitive members are transferred onto an intermediate transfer belt so that the color images can be combined with each other. In an image forming apparatus of the tandem type, for example, an optical scanning apparatus which comprises a single deflector (polygon mirror) for scanning four beams concurrently to write images on the four photosensitive members is provided.
In an optical scanning apparatus of this type, on each side of a deflector, an upper scanning system and a lower scanning system are provided. Three types of the upper and lower scanning systems are known. One is a type composed of lenses commonly used for the upper scanning system and the lower scanning system. Another is a type composed of lenses exclusively used for the upper system and lenses exclusively used for the lower system. The other is a type composed of one or more lenses commonly used for the upper and lower scanning systems, lenses exclusively used for the upper system and lenses exclusively used for the lower system. Of these three types, the type composed of lenses commonly used for the upper and lower systems is the most advantageous in view of cost.
However, when a single set of lenses is to be used commonly for the upper and lower systems, the optical surfaces of the lenses must be shaped symmetrically in a sub-scanning direction. Accordingly, when beams are incident to the optical surfaces of the lenses at an angle with respect to the sub-scanning direction, on receiving surfaces, writing lines in the sub-scanning direction have remarkable bows. Then, if the lenses are designed to correct the bows, other aberrations will become heavier, and the beam performance will be worse. If the bows are kept unsolved, because the bow occurring in the upper system and the bow occurring in the lower system are different in curving direction, an image having color displacement in the sub-scanning direction will be formed.
Japanese Patent Laid-Open Publication No. 2002-202472 discloses an optical scanning apparatus wherein the numbers of diverting mirrors provided respectively on the right side, on the left side, on the upper side and on the lower side of a deflector are regulated so that bows caused by linear expansions of a housing with changes in temperature will be uniform in curving direction. According to this publication, the number of diverting mirrors provided on the right side of the deflector and the number of diverting mirrors provided on the left side of the deflector are respectively even and odd or respectively odd and even. The number of diverting mirrors provided on the upper side of the deflector and the number of diverting mirrors provided on the lower side of the deflector are both even or both odd. However, in the optical scanning apparatus according to this publication, lenses are exclusively used for each optical system, and therefore, the apparatus is costly. Also, since beams are not incident to the deflector at an angle, the deflector must be made thick, which further increases the cost. While the number of diverting mirrors on the right side of the deflector and the number of diverting mirrors on the left side of the deflector are respectively even and odd or respectively odd and even, the number of diverting mirrors above the deflector and the number of diverting mirrors below the deflector are both even or both odd. Therefore, although bows caused by linear expansions of a housing are uniform in curving direction, bows in four colors caused by the design of optical elements are different in curving direction.
According to Japanese Patent Laid-Open Publication No. 2004-70107 discloses an optical scanning apparatus wherein the number of diverting mirrors provided on the right side of the deflector and the number of diverting mirrors provided on the left side of the deflector are both even or both odd and wherein the number of diverting mirrors provided above the deflector and the number of diverting mirrors provided below the deflector are respectively even and odd or respectively odd and even. Accordingly, in this optical scanning apparatus also, bows in four colors are different in curving direction.